Field of the Invention
The invention concerns a method, a magnetic resonance apparatus and an electronically readable data carrier for the automatic assignment of a spin species to a combination image that was obtained from at least two magnetic resonance datasets of a multi-contrast measurement.
Description of the Prior Art
Magnetic resonance (MR) technology is a known modality that can be used to generate images of the inside of an examination object. In simple terms, the examination object is positioned in a magnetic resonance scanner in a strong, static, homogeneous, constant magnetic field, also called a Bo field, with field strengths of normally 0.2 Tesla to 7 Tesla and more, such that nuclear spins in the object are oriented along the constant magnetic field. To trigger nuclear spin resonances, radio-frequency excitation pulses (RF pulses) are radiated into the examination object. The triggered nuclear spin resonances are measured as so-called k-space data, and on the basis thereof MR images are reconstructed or spectroscopy data are determined. For spatial encoding of the measurement data, rapidly switched magnetic gradient fields are overlaid on the constant magnetic field. The recorded measurement data are digitized and stored in the form of complex numeric values in a k-space matrix. An associated MR image can be reconstructed from the value-filled k-space matrix, for example by a multidimensional Fourier transform.
Various MR techniques are known for separating signals from different spin species, for example spin in a water environment and spin in a fat tissue environment, as well as other environments, into recorded MR data or for suppressing the signals from particular spin species. These techniques include so-called multi-contrast measurements. One such technique is the Dixon technique, in which at least two MR datasets are recorded at different echo times in each case following an excitation, and are therefore recorded with different contrasts. In this case the echo times are selected such that the relative phase position of different spin species of the signals contained in an MR dataset is different in the various recorded MR datasets. With knowledge of the respective phase positions it is possible to extract combination images from the recorded MR datasets reconstructed MR datasets, which for example represent signals from only one spin species.
In principle a Dixon technique can be implemented using different sequence types, for example with (turbo) spin echo sequences, SSFP sequences (SSFP: “steady-state free precession”) or gradient echo sequences, and it therefore has a wide range of applications.
To determine a combination image the reconstructed MR images are generally set off against one another pixel by pixel. In order to eliminate unknown phase portions, the absolute signal portions of the pixels are taken into consideration, which means the results are not unambiguous if a subtraction takes place, since
          S        =      {                                                                                        Sp                  ⁢                                                                          ⁢                  1                                -                                  Sp                  ⁢                                                                          ⁢                  2                                            ;                                                                          Sp                ⁢                                                                  ⁢                1                            >                              Sp                ⁢                                                                  ⁢                2                                                                                                                          Sp                  ⁢                                                                          ⁢                  2                                -                                  Sp                  ⁢                                                                          ⁢                  1                                            ;                                                                          Sp                ⁢                                                                  ⁢                2                            >                              Sp                ⁢                                                                  ⁢                1                                                        ,      wherein S represents the value of the signal and Sp1 and Sp2 represent the signal components of two different spin species, e.g. water and fatty tissue. All that can be stated is which combination image represents the dominant spin species, namely the one which is determined by an addition of signals from the at least two captured MR datasets, and which combination image represents the subordinate spin species, namely the one which is determined by subtracting signals from the at least two captured MR datasets. The most frequent application of a Dixon technique is water/fat separation, in which a water image and/or a fat image is to be determined as a combination image.
Although the result of a Dixon technique is therefore the combination images referred to, which represent signals from only one spin species, it is not however readily apparent which combination image obtained is to be assigned to which spin species. When a fat/water separation is desired, it is not therefore possible to say which of the determined combination images is the fat image and which the water image. The correct assignment still has to be selected first.
If the derived combination images are determined by optimization, in which there may be several local minima, an ambiguity of this type likewise exists. This is because although all the minima are known, it is still not possible to know which one is the one sought (in the simplest case the values of the optimization function are identical for all minima—the minima are thus degraded). However, the minima can, as described, be characterized e.g. by the dominant species. The same applies if the combination images are determined as a result of phase unwrapping with different start points. In each case the correct assignment is initially still unknown.
If several Dixon recordings are taken, so that several sets of MR images of just one spin species are obtained, it can happen that the assignment has taken place differently in the different sets, which means MR images from different sets, which in fact represent the same spin species, are assigned to different spin species.